Measuring Life’s Limits in the Ocean

Sea life may not lack water for survival, but marine organisms do need nutrients such as nitrate, phosphate, silicate and iron. Researchers, therefore, want an accurate, high-resolution profile of nutrient concentrations in the open ocean, which is a challenge given the five or more orders of magnitude such concentrations span throughout the depths.

A 3-D schematic of the second-generation Spectrophotometric Elemental Analysis System instrument shows the pressure housing that contains the battery and heater on the lower right, the three pumps in the center, and the housing containing the lamp, spectrometer and related hardware on the top left. Courtesy of Eric A. Kaltenbacher, SRI International. Now a group of scientists at the University of South Florida and at SRI International, both in St. Petersburg, has demonstrated phosphate and nitrate measurements over a large concentration range with the team’s second-generation Spectrophotometric Elemental Analysis System (SEAS II). Marine science professor Robert H. Byrne noted that SEAS II was not built overnight. “We have developed this technology over approximately 10 years of effort and feel that it is now mature enough to be transferred and made available to the wider scientific and commercial communities.”

A nitrite profile for the North Pacific Subtropical Gyre shows multiple peaks observed using SEAS II that are difficult to see using standard methods. Courtesy of Lori R. Adornato, SRI International.If nitrate or phosphate is in short supply in the open ocean, the concentration at the surface may be in the low nanomolar range, while at depth it is measured in micromolars. However, techniques that measure the higher concentrations accurately do not produce results at the lower ones.

The investigators turned to a technique called long-path-length absorbance spectrophotometry to overcome this problem. As with standard absorbance spectrophotometry, the long-path-length version monitors absorbance of light at specific wavelengths. That information, combined with known absorptivity, enables determination of the concentration of specific nutrients. The newer method extends sensitivity because it expands the absorbing path length.

At one time, it would not have been practical to have a long path length in an ocean-going instrument. Advances in materials now allow the construction of flexible optical cells with complete internal reflection, thereby enabling the cell to be wound inside a casing. For SEAS II, the group used a DuPont polymer to make a liquid-core waveguide up to 10 m long. The scientists placed the waveguide cartridge inside a 50-in. anodized aluminum housing, along with electronics and a battery for eight-hour operation.

The SEAS II instrument, secured in a custom-made frame along with other sensors, is retrieved after a run during a cruise in the Gulf of Mexico. Courtesy of Jenny Wollschlager and Eva Romera Luna, Eckerd College, St. Petersburg, Fla.They used a spectrometer from Ocean Optics Inc. of Dunedin, Fla., with a lamp from Avantes Inc. of Boulder, Colo., for a light source. The device also contained a heater and pumps. In operation, SEAS II is submerged and stores data until it surfaces.

After first determining which reagents functioned best, the researchers measured the nitrate profile in the Gulf of Mexico. Their results agreed well with those of other methods. They did the same for phosphate and nitrite, again with good agreement.

Byrne noted that, although the results agreed with those of other methods, there were some unexpected findings. “We were surprised by the nuances in the nitrite profiles, which were on such small scales that they could not be resolved prior to the development of this technology.”

Plans call for the development of real-time communication capabilities with the instrument and a reduced-cost system suitable for long-term stationary deployment. One use envisioned for such an instrument would be to monitor carbon system parameters, thereby providing information to better explain the impact of increasing atmospheric carbon dioxide.